Dynamics of malaria parasitaemia associated with febrile illness in children from a rural area of Madang, Papua New Guinea.

Active community and self-reporting surveillance techniques have been used to describe the dynamics of febrile illness and associated malaria infection in children aged 2 to 15 years from a rural area of Madang Province, Papua New Guinea (PNG). Both history of fever and fever in association with parasitaemia appeared to be reliable indicators of malaria morbidity in this endemic area. Parasite density was observed to be a major determinant of mild malarial disease at both the population level and within an individual. Age-specific prevalence of febrile illness correlated with age-specific patterns of parasite density but not of parasite prevalence. Seasonal changes in fever incidence correlated with parasite density. The transition from afebrile to febrile state within an individual was generally associated with an increase in parasite density. Surveillance and self-reported febrile cases (which differ in severity on the basis of the perceived need for treatment) could be distinguished on the basis of parasite density. Thus surveillance techniques divide clinical malaria in rural PNG into 'mild' and 'very mild' forms. The age-specific pattern of decline of prevalence of malaria-associated febrile illness and parasite density is best explained by induction of strain-specific anti-disease immunity upon infection with a given strain of Plasmodium falciparum. The fever threshold in self-reporting febrile cases was seen to decrease with age and can be explained by an age-specific decline in anti-toxic immunity.